The rapid expansion of the number of cellular radio telephones coupled with the desire to provide additional services has prompted the development of a new standard. The standard suggests an increase in system capacity over the current analog system through the use of digital modulation and speech coding techniques. The standard uses a time division multiplex (TDM) system which splits the current channel into six signal packets of which three are currently in use. A packet is a burst of information characterized by sequentially encoded symbols for the intended receiver. The linear modulation technique to transmit the digital information within the channel is .pi./4 DQPSK (differential quadrature phase shifted keying).
The use of .pi./4 DQPSK linear modulation in the U.S. Digital Cellular system provides spectral efficiency allowing the use of 48.6 kbps channel data rates. .pi./4 DQPSK transmits the data information by encoding consecutive pairs of bits, commonly known as symbols, into one of four phase angles (.+-..pi./4, .+-.3.pi./4) based upon gray encoding. These angles are then differentially encoded producing an 8 point constellation.
Cellular systems operate in the existing 800 MHz band. Radio propagation at these frequencies is generally characterized by three types of distortion: time dispersion distortion, multipath distortion and lognormal distortion. Time dispersion distortion of a received signal occurs when a transmitted signal is received via more than one propagation path each having a different path length. Measured received signals having time dispersion distortion typically have a strong first component and multiple components that are generally lower in amplitude for larger delays. Time dispersion distortion of the received signal is usually found in an environment where a large reflecting source, such as a mountain, is present. A mobile radio in this environment receives the signal from a fixed source transmitter and the delayed signal from the reflecting source. The time delay between the reception of the two signals results in time dispersion distortion.
Multipath distortion is characterized by many rays of the same signal having different energy levels reaching the receiver at the same time. The number, phase and intensity of the signals received by the receiver in a multipath environment may vary over time as a result of repositioning of the receiver, or of the objects from which a transmitted signal is reflected. As a result, the phase and signal level of a received signal varies over time. This variance is referred to as "fading" of the signal. The resultant signal strength and rate of change of signal strength at the receiver is predominantly determined by how rapidly the receiver is moving through its environment, and the frequency of the channel being used. For instance, in the cellular frequency band, and when a cellular radio telephone is positioned in a vehicle traveling at 60 mph, the signal strength of the received signal can vary by approximately 20 decibels during a 5 millisecond period.
In the case of time dispersion and multipath distortion, two received signals transmitted from the same source which are 180 degrees out of phase effectively cancel each other out. The received signal's intensity approaches a null and the rate of change of the received signal intensity over time is rapid. Since the received signal strength intensity is low, the modulated information can be corrupted by noise present in the channel. A signal corrupted by noise can alter the state of the demodulated information thereby causing the receiver to detect wrong information.
Lognormal distortion of a received signal occurs when the distance between the transmitting source and the receiver increases thereby causing a logarithmic decrease in the signal strength at the receiver. The distance at which lognormal distortion begins depends upon the transmitter's signal power and the receiver's sensitivity. As the distance between the transmitting source and the receive increases, the received signal strength intensity may decrease to a level whereby the modulated information is corrupted by noise present in the channel. As with time dispersion and multipath distortion, a signal corrupted by noise can alter the state of the demodulated information thereby causing the receiver to detect wrong information.
Recovering a signal packet having time-varying signal intensity is feasible when the packet is relatively short. For example, variation of the signal intensity over a packet having a 0.5 millisecond duration is usually not significant enough to alter the state of the information in the packet. If the entire packet is lost in noise, the performance of the system would not be substantially degraded. The packet with short duration contains less information than longer duration packets. The signal's intensity is considered to be constant over the duration of the packet while the information in the packet is recovered.
However, systems which specify a signal packet having a relatively long duration, for example, 6.66 millisecond duration in the U.S. Digital Cellular system, variation in the signal strength intensity can be significant. Variations can cause the signal intensity to approach the noise floor of the channel thereby corrupting the information in the packet thereby causing the receiver to recover wrong information.
Thus, a formidable challenge is to provide a system for recovering information in a time-varying signal packet having a long duration.